Tomterrific wrote:1)Wood is super strong in compression....
2)Where problems arise is with the structure racking. A book shelf is a good example. A simple diagonal member turns the weak square into a triangle and the shelf does not rack....
3)The open back is another matter. Gussets top and bottom will triangulate part of the opening....
4)The 5 foot span of the roof is another problem. You will need a stiff roof and the wood will not be in pure compression so weak....
5)Gussets at every wall batten would also help....
6)I can't imagine how irritating having someone sleeping 3 feet above my head....
1)* Whether you use a simple sheet of 3/4" plywood as walls (as in my build), or use thinner ply in a composite sandwich (like
tony.latham), laying a similar sheet atop the upright edges of the walls will support up to 875 lbs (uniform loading, at least in my calculations) without appreciable sag.
2)* The diagonal bracing is a good idea for thin plywood, or even one with thin foam core composite, but my trailer's 3/4" solid plywood , and Tony's thicker composite, act as a stressed skin panel. Just as strong.
3) * There's really no open back to the squareback design, with the front sloped wall closing off that end (mine is also braced with extra brackets and oak boards across and on the sides...tied into the doorframe bracing), and the middle having the rear bulkhead holding upthe roof, while the short 22" overhang (18" in the OP's plan) is negligable (especially if the rear of the trailer is a closed wall...with a hatch opening cut-out). Plus, the angle brackets I used liberally are gusseted, and the corner brackets at every corner, top/bottom, are thick steel with side supports.
4)* Answered in reply
1. Plus, a single beam/spar located midway between front slope and rear bulkhead will make the sag almost nil.
5) * Answered in reply
3; the Simpson StrongTies I used are gusseted and the corner brackets are made with thicker/stronger material to prevent this (the carriage bolts securing these are backed with 1.25" diameter fender washers to prevent pull-thru, while prodigious amounts of PL adhesive further prevent movement on all interfaces).
- angle & corner brackets.png (197.71 KiB) Viewed 4691 times
6)* Use a layer (or two) of sound-deadening material, either on the cabin ceiling, or on top, under the rooftop tent. Good at blocking sound/vibration transmission on cars, this stuff is also (most brands) also waterproof and usually self-adhesive.
Dynamat here's one type specifically for decking
http://www.dynamat.com/automotive-and-transportation/automotive-restoration/dynadeck/#:~:text=As%20DynaDeck%20is%20completely%20weatherproof,so%20no%20adhesive%20is%20needed.is known for effectiveness, and I've used it before. Here's alternatives compared
https://aquietrefuge.com/dynamat-alternatives/